1. Field of the Invention
The present invention generally relates to a Faraday rotator glass composition and more specifically to an ultraviolet (UV:200 to 400 nm) transparent Faraday rotator glass composition.
2. Description of the Related Art
Glass compositions which exhibit the Faraday rotator phenomenon are well known to those skilled in the art. Examples of such Faraday rotators are discussed in James L. Dexter et al., Ultraviolet Optical Isolators Utilizing KDP-isomorphs, Vol. 80, No. 2 OPTICS COMMUNICATIONS pp. 115-118 (Dec. 15, 1990), incorporated herein by reference.
Some optical materials are optically active wherein rotation of plane polarized light is observed for such light passing through such optical material. However, many glasses do not rotate plane polarized light unless the glass is placed in a magnetic field when the plane polarized light is passed through it. This rotation of plane polarized light passing through the glass is called the Faraday effect.
There are two important measurable characteristics of Faraday materials that determine their usefulness. These are the Verdet constant, V, and the optical transmission, OT, of a given material at a given wavelength, .lambda., of light. In general, the relationship between the Verdet constant (V) and the optical transmission (OT) is as follows: EQU V.congruent.k.sub.1 /k.sub.2 (.lambda.).sup.2 .congruent.k/OT
where k, k.sub.1, and k.sub.2 are numeric constants, and .lambda. is the wavelength of light at which the Verdet constant, V, and the optical transmission, OT, are determined. An optimal Faraday material would have a high V constant and a high OT at a desired wavelength, .lambda.; however, materials that tend to have high V constants also tend to have high absorption in the UV range, or stated alternatively, a low OT. Conversely, materials in the UV range with high OT have low V constants.
Optical isolators using Faraday rotators have not been widely used in the ultraviolet (UV:200-400 nm) region because materials with desirable Faraday rotating characteristics, denoted by a high Verdet constants (V.congruent.k.sub.1 /k.sub.2 .lambda..sup.2) at a desired wavelength, .lambda., tend to have low transmission in the UV region of the absorption spectrum. Id. at 115. According to Dexter et al., " . . . [m]aterials with large Verdet constants tend to have large absorption in the ultraviolet, while materials with good UV transmission have relatively small Verdet constants." Id.
Several materials have been used as Faraday materials and the advantages and disadvantages of those materials are cited in the table below:
__________________________________________________________________________ MATERIAL WAVELENGTH ADVANTAGES DISADVANTAGES __________________________________________________________________________ Water 308 nm, 248 nm Impurities in the water; Low Verdet constant Fused Silica 253.7 nm, 248 nm V = 1710 deg/T-m at Impurities present 253.7 nm for suprasil; drastically alter the V = 1920 deg/T-m at Verdet constant values; 248 nm for fused silica Verdet constant very sensitive to small changes in composition Potassium dihydrogen 248 nm, 222 nm, 193 nm, Superior to water and Can be used only down to phosphates (KDP & KDP 190-350 nm fused silica 190 nm with high isomorphs) OT; Material is hygroscopic so requires dry cell which adds 4 unwanted surfaces; Materials are soft thus cannot be polished to high optical grade flatness and cannot use anti- reflection coatings Ammonium dihydrogen 308 nm, Superior to water and Larger Verdet constant arsenates (ADA); also fused silica; V = 1526 than the phosphates but Potassium dihydrogen 190-350 nm deg/T-m at 351 nm for have high OT only down arsenates (KDA & KDA ADA; V = 2328 deg/T-m at to 260 nm; Material is isomorphs) 308 nm for ADA hygroscopic so requires a dry cell adding 4 unwanted surfaces; Materials are soft thus cannot be polished to high optictal grade flatness and cannot use anti- reflection coatings __________________________________________________________________________
As noted in the table above, of the Faraday materials which have been used at UV wavelengths, the KDP-isomorphs are superior to both fused silica and water. However, better materials are desired due to the limitations of the isomorphs resulting from the material softness and hygroscopic nature. A characteristic common to the KDP-isomorphs is the relative softness of the crystals, which prevents polishing of the optical surfaces to a high surface flatness. Typically, the best flatness which can be achieved across the surface of the KDP-isomorphs is one-tenth (1/10) of a wavelength at 632.8 nm. Surface figures of one-fortieth (1/40) of a wavelength, or better, at 632.8 nm are desired to produce acceptable surface figures at UV wavelengths. Poor surface figures contribute to excessive wavefront distortion and scattering losses, particularly in the UV region. Further, due to their softness, the crystals cannot be supplied with anti-reflection coatings. All of these problems limit the practicality of the KDP-isomorphs as UV Faraday rotator materials. Practical Faraday rotators at wavelengths shorter than 400 nm are needed for operating in optical isolators for uses with lasers operating in the UV wavelength region. There is a strong need for better UV Faraday rotating materials that are harder, less hygroscopic than the KDP-isomorphs, have higher V constants and have higher OT in the UV wavelength region.